Method of producing enameling iron,and enameling iron compositions and articles

ABSTRACT

1. A METHOD OF PRODUCING SAG-RESISTANT ENAMELING IRON SHEET HAVING RIMMING STEEL QUALITY SURFACES AND ENHANCED RECRYSTALLIZATION RESPONSE, COMPRISING: (A) AIR MELTING AND REFINING AN IRON BASE MELT TO A COMPOSITION CONSISTING ESSENTIALLY OF, BY WEIGHT PERCENT:   CARBON-----------.05 TO ABOUT0.010%. MANGANESE ------ 0.07 TO 0.12%. SULFUR --------- UP TO 0.04%. SILICON -------- UP TO 0.10%. TITANIUM ------- UP TO 0.01%. IRON ----------- BALANCE, EXCEPT FOR OXYGEN AND INCIDENTAL STEELMAKING IMPURITIES.   (B) DEOXIDIZING THE REFINED COMPOSITION BY VACUUM CARBONDEOXIDATION AND FOR A TIME AND AT A SUBATMOSPHERIC PRESSURE SUFFICIENT TO REDUCE THE OXYGE CONTENT OF THE MELT TO LESS THAN0.02% AND THE CARBON CONTENT TO LESS THAN 0.04%&lt; (C) WITHOUT ANY ADDITION OF TITANIUM, CASTING THE VACUUM-CARBON:DEOXIDEZED COMPOSITION OF STEP (B) INTO A ROLLABLE ARTICLE, (D) ROLLING THE ARTICLE IN A PLURALITY OF ROLLING STEPS INCLUDING A PLURALITY OF SUCCESSIVE HOT ROLLING STEPS DEFINING A HOT ROLLING ZONE AND WHEREIN AT LEAST THE FINAL ROLLING STEP IS A HEAVY COLD ROLLING STEP OF AT LEAST ABOUT 50% REDUCTION TO PROVIDE A SHEET, (E) HEATING SAID COLD ROLLED SHEET IN A SINGLE SUBCRITICAL ANNEALING TREATMENT TO ESSENTIALLY COMPLETELY RCRYSTALLIZED SAID SHEET.

Oct. 22, 1974 w. c. LESLIE 3 3,

METHOD OF PRODUCING ENAIELING IRON, AND ENAIELING IRON COHPOSITIONS AND ARTICLES Filed larch 9 1972 L A ALAI III 10 minutes TIME.

RECRYSTALLIZATION OF Fe Mn 0 ALLOYS AT 1100 F AFTER NOT ROLLING AT 2100 F COOLING AND ROLLING TO 60% Reduction REDUCTION O O l TIME AT ANNEALING TEMPATURE, minutes RECRYALLIZATION OF LOW-CARBON STEELS AFTER 607n COLD REDUCTION United States Patent vMETHOD OEPRODUCING ENAMELING IRON, 'AND ENAMELING IRON COMPOSITIONS AND ARTICLES V William C. Leslie, Franklin Township, Westmoreland County, Pa., assignor to United States Steel Corporation r Continuation-impart of application Ser. No. 842,829, July 11, 1969, which is a continuation-in-part of application Ser. No. 593,799, Nov. 14, 1966, both now. abandoned. This application Mar. 9, 1972, Ser. No. 233,280

Int. Cl. C21d 7/14, 9/46 US. Cl. 148-2 2 Clail ns ABSTRACT OF THE DISCLOSURE Method of producing enameling iron articles comprising vacuum carbon deoxidizing an air melted steel melt comprising at least 0.05% carbon, 0.04 to-0.12% manganese and usual oxygen in equilibrium with the carbon content, to reduce carbon to less than 0.04% and oxygen'to less than 0.03%, casting and forming from the melt an article which is recrystallizable after heavy reduction by subcritical annealing without normalizing, and articles so produced.

RELATED APPLICATIONS This application is a continuation-in-part of application Serial No. 842,829, filed July 11, 1969, now abandonedywhich in turn is a continuation-in-part of application Ser. No. 593,799, filed Nov. 14, 1966, now abandoned; in the name of William C. Leslie, for Composition of Enameling Iron.

BACKGROUND OF THE INVENTION Conventional enameling iron is characterized by a low carbon content and low manganese content. A typical Enameling iron is a rimming variety of very low carbon steel. The low carbon limitation is necessary to achieve good enameling properties, i.e. freedom from defects such as black specks, eye-holing, fish-scaling and the like. Low manganese is required to impart sag resistance, the latter being essential to avoid deformation of the material at the high firing temperature (1200-1600 F.), incident to the porcelain enameling process.

In the usual air melting process of manufacturingv enameling iron, it is necessary to refine the melt extensively inorder to achieve the requiredlow carbon and manganese contents. Since sulfur is not readily reduced below 0.01 percent by therefining process, such material is hot short by reason of its low manganese content (hence high sulfur-to-manganese ratio), and, in the rolling of steel mill products comprising such compositions, the rolling must be interrupted to allow the products to cool through thetemperature range between about 1950' and 1650 F. a w

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The most serious limitation of conventional enameling iron, however, is its resistance torecrystallization during annealing after cold rolling. This is such that it cannot be consistently softened by the usual sub-critical temperature annealing technique, and resort must be made to the more expensive continuous normalizing process. Moreover, if minimum hardness is desired, the continuous normalizing must be followed by a box annealing treatment.

In the air-melt refining of enameling iron melts to the required low carbon and manganese levels, the oxygen content of the melt undergoes a large increase, e.g. to 0.06 to 0.10% as abovestated.

Since the advent of commercial-scale vacuum-treatment equipment in the steel industry, this technique has also been utilized in processes for the production of enameling irons. For example, Ohtake et al. US. Pat. 3,183,078, teach the production of killed enameling irons having the usual range of manganese for enameling irons, i.e. up to 0.30%. together with low carbon (under 0.02%), by the use of a combination of aluminum and titanium addi tions following the vacuum carbon deoxidation of a somewhat higher carbon-containing (under 0.04% carbon) melt. By forming compounds of aluminum, titanium and sulfur, the hot shortness caused by the latter element is said to be reduced. so that manganese may be decreased, e.g. to under 0.05%.

However, such additions of aluminum and titanium to produce. a fully killed steel have several disadvantages. First, air-melt refining of the initial melt to carbon levels as low as 0.03 to 004% results in a disproportionately higher oxygen content than is the case if refining is sto ped at a higher carbon level, eg. in the range of 0.05 to 0.10%. Consequently. when such low carbon (below 0.05%) melts are subjected to vacuum carbon deoxidation, reduction of carbon to the desired final level, e.g. under 002%, results in substantial residual equilibrium oxygen. If lowest oxygen content and hence greatest steel cleanliness is desired, the oxygen must be removed with deoxidizing elements, such as the aluminum, titanium additions of Ohtake et al. Such additives are costlv and, moreover, form various compounds, as nitrides, sulfides, oxides, etc. which. even in small amounts, form undesirable inclusions affecting the surface and other qualities of the final product.

Further, such prior art fully killed steels undergo no rimming action whatsoever during teeming and, accordingly, do not partake of the surface quality and yield enhancement of unkilled steels.

PREFERRED EMBODIMENT OF THE INVENTION It has now. been found that enameling iron stock having improved cleanliness. good enamelin and sag-resistant properties, and which stock is readily and.substantially completely recrystallizable by means of a single annealing treatment following cold reduction of at least 50%, can be produced by subjecting the hereinafter described melt .to vacuum carbon deoxidation to decrease the oxygen content thereof to below 0.03%. These highly desirable properties (contrary to the teaching of Ohtake et al.) are achieved without the purposeful addition of titanium either in the melt or subsequent to the vacuumcarbon-deoxidation treatment. The instant method, therefore, comprises providing a metal melt essentially comprising by weight percent:

incidental steelmaking residuals.

and subjecting said melt to vacuum carbon deoxidation for a time and pressure sufiicient to decrease the carbon content to less than 0.04% and the oxygen content thereof to less than 0.03%. Unlike the method of Ohtake et al. this unkilled degassed composition is then cast without any further addition of titanium and then rolled to the form of enameling iron sheet. Following a cold reduction of at least 50% the sheet is recrystallized by a single subcritical annealing.

DESCRIPTION OF THE INVENTION FIG. 1 is a graph showing the effects of variation in manganese content on the recrystallization response of heavily cold worked alloys of Fe-Mn-O when annealed at sub-critical temperature for various lengths of time, and

FIG. 2 is a similar graph but showing the response of low carbon steel at two levels of manganese and two levels of oxygen.

Referring first to FIG. 1 wherein the effect of manganese on the recrystallization behavior has been isolated, the curves A, B, C and D represent respectively the response to annealing at 1100' F. after a cold reduction of 60% of four specially-prepared iron-mauganese-oxygen alloys of the following analysis:

The alloys were prepared to duplicate the amount of oxygen normally found in conventional enameling irons; all had the same treatment prior to annealing, i.e. were hot rolled at 2100" F., air cooled, cleaned and then cold reduced 60%. It is apparent from the curves that the addition of a small amount of manganese, about 0.1%, markedly suppresses the recrystallization but that increasing manganese to about 0.3% restores the normal behavior. The above data appear to confirm the opinion generally held by workers in this art that the low manganese content of enameling iron is responsible for its resistance to recrystallization. However, I have discovered that oxygen content of the iron is a critical factor in this regard.

The latter is illustrated by FIG. 2 wherein curves 1, 2 and 3 represent respectively the annealing response of three enameling irons of the following analysis:

The above steels were processed identically and after 60% cold reduction, samples of each were given subcritical anneals for various periods of time. With the exception of oxygen content, steels 1 and 2 are substantially the same chemically. Steel 1 is a conventional enameling iron analysis and exhibited the typical annealing response of such material, i.e. recrystallized about 30% after about 17 hrs. at 1050 F. Steel 2, containing less than 0.01% oxygen, however, recrystallized about 90% in the same period and at the slightly lower temperature of 1000 F. The complete recrystallization of steel 3 at 1000 F. is expected in view of its high manganese content and apparently reduction in oxygen is not necessary if sufficient manganese is present.

As is apparent from the foregoing, response to recrystallization at sub-critical temperatures is readily obtained by increasing the manganese content, but as mentioned earlier, this is accomplished only at the expense of sag resistance. However, the discovery of the critical role of oxygen in the presence of low manganese aifords achievement of good annealing response without the loss of sag resistance. The mechanism of the above disclosed effect of limiting oxygen in the low manganese composition of enameling steels is not fully understood at present. It may be speculated that excessive oxygen results inthe formation of manganese-oxygen complexes at the sites of manganese atoms in the iron lattice in sufficient number to interfere with recrystallization. However, I do not wish to be limited to such explanation, since it is suflicient to practice my inventions to know that oxygen less than 0.02 to 0.03% preferably about .005%, in combination with manganese between 0.04%, preferably 0.07% and 0.12% provides an enameling iron which is sag resistant, readily annealable and nearly free of inclusions.

Since stock for porcelain enameling requires a clean metal, i.e. substantially free of inclusions, achievement of low oxygen by treatment with strong deoxidizers such as, for example, aluminum or titanium, must be avoided.

Accordingly, the following illustrates a preferred practice: a bath of molten iron is refined to a carbon content of .052%, i.e. about 3 points higher than desired in the finished product. It is thus contemplated to'refine the iron to a carbon content of about 0.05 to 0.10%, preferably 0.05 to 0.06%. Refining may be by any conventional, e.g. open hearth or basic oxygen, practice for making lowcarbon rimming steel. Upon completion of refining, the manganese content of the heat is adjusted to about 0.1% after which the metal is subjected to a vacuum carbon deoxidation treatment by so-called stream degassing in which the steel is poured from a tundish into a receiving ladle in an evacuated chamber maintained at an absolute pressure of about 3 mm. of mercury to lower the oxygen content to 0.02% and the carbon content to about 0.02%. Pressures below about 5 mm. of mercury are desirable. Such operation is economical of both time and heat and readily lowers the oxygen content to below 0.02%, generally to about 0.1%, and at the same time reduces the carbon content below 0.03%, generally to about .02% or slightly less. Other types of degassing equipment may be used. An example of a vacuum-carbondeoxidized heat of this invention is as follows:

Cu Ni Gr Mo 0 With regard to others than the aforesaid essential elements in the composition, sulfur, phosphorus and nitrogen are employed in amounts normally present in open hearth steels.

Copper may be present in the amounts normally used to impart atmospheric corrosion resistance, i.e. up to about 0.3%. Nickel, cobalt, chromium, vanadium and molybdenum normally will be present in only trace amounts; however, small amounts of these elements, e.g. up to about 0.1% have no adverse effect insofar as the primary objectives of the present invention are concerned and accordingly, could be added for their known special eifects if desired.

In general, the manufacturing procedure is aimed to produce a clean enameling iron of the following composition:

Broad range Preferred range Ohtake et a1. composition is at least five times that of the maximum contemplated by the instant composition. Larger amounts of manganese, within its range, are used when sulfur is on the high side of its range. Thus, in accordance with the inventive process, the benefits of low carbon and oxygen are realized in an unkilled steel free of compound-producing deoxidizers, and the detrimental eifects of sulfur on hot shortness is also avoided without the necessity of adding an expensive and detrimental sulfur-binding element such as titanium.

Processing of the new iron from ingot to sheet is conventional except for the following:

(1) Hot rolling may be conducted without the customary interruption in the temperature range 1950-1650 F. For some reason, as yet unknown, the new iron is considerably less hot short than the conventional.

(2) The necessary heat treating following cold reduction is accomplished by regular box annealing practices. The elimination of the necessity for continuous normalizing aifords a substantial saving.

While I have shown and described only certain embodiments of my invention, it is obvious that modifications can be made without departing from the scope of the appended claims.

I claim:

1. A method of producing sag-resistant enameling iron sheet having rimming steel quality surfaces and enhanced recrystallization response, comprising:

(a) air melting and refining an. iron base melt to a composition consisting essentially of, by weight percent: I

Carbon .05 to about 0.10%. Manganese 0.07 to 0.12%. Sulfur Up to 0.04%. Silicon Up to 0.10%. Titanium Up to 0.01%. Iron Balance, except for oxygen and incidental steelmakin g impurities.

(b) deoxidizing the refined composition by vacuum carbon deoxidation and for a time and at a subatmospheric pressure sufficient to reduce the oxygen content of the melt to less than 0.02% and the carbon content to less than 0.04%, 1

(0) without any addition of titanium, casting the vacuum-carbon-deoxidized composition of step (b) into a 'rollable article,

(d) rolling the article in a plurality of rolling steps including a plurality of successive hot rolling steps defining a hot rolling zone and whereiri' at least the final rolling step is a heavy cold rolling step of at least about reduction to provide a sheet,

(e)"heating said cold rolled sheet in a single subcritic'al annealing treatment to essentially completely recrystallize said sheet.

2. The method of claim 1, wherein the vacuum carbon deoxidation of step (b) is sufiicient to reduce the oxygen content' to a value of 0.01 max. and the carbon content to a value of from 0.01 to 0.02%.

References Cited UNITED STATES PATENTS 2,065,392 12/1936 Porter 148-12 3,239,390 3/1966 Matsukura 148-12.1 3,178,318 4/1965 Shimizu et a1 148-2 3,230,074 1/ 1966 Roy et al. -49 3,522,110 7/1970 Shimizu et a1. 148-12 FOREIGN PATENTS 1,018,171 1/1966 Great Britain 75-49 868,938 5/ 1961 Great Britain 75-49 1,055,946 1/ 1967 Great Britain 75-49 282,405 12/ 1964 Netherlands 75-49 OTHER REFERENCES Trans. of ASM, vol. 58, 1965, pp. 672-686. Metals and Alloys, vol. 1, No. 15, September 1930, pp. 71; and 7'13.

CHARLES N. LOV-ELL, Primary Examiner U.S. Cl. X.R. 75-49; 148-12 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,843,415 Dated October 22, 1974 Inventor(s) C. Leslie It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4,, line 34, "0.1%" should read .0.01Z

2 line 40, "0 should read 0 Signed and sealed this 14th day of January 1975.

(SEAL) Attest:

McCOY M. GIBSON JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents FORM PO-1050 (10-69) uscoMM-oc 60378-P69 u.s sovzgunsu'r vnm'rms OFFICE: 930 

1. A METHOD OF PRODUCING SAG-RESISTANT ENAMELING IRON SHEET HAVING RIMMING STEEL QUALITY SURFACES AND ENHANCED RECRYSTALLIZATION RESPONSE, COMPRISING: (A) AIR MELTING AND REFINING AN IRON BASE MELT TO A COMPOSITION CONSISTING ESSENTIALLY OF, BY WEIGHT PERCENT: 